1. Field of the Invention
This invention relates to magnetic resonance imaging systems. In a primary application this invention involves a pulsed magnetic field used to polarize the magnetic moments of an object. As the pulsed field collapses, a selection gradient is used to cause the magnetic moments to vary spatially, providing slice selection.
2. Description of Prior Art
The basic concepts in magnetic resonance are described in a series of papers in the June 1980 series of the IEEE Transactions on Nuclear Science, Vol. NS-27, pp 1220-1255.
Exisiting MRI systems have a large number of theoretical and practical problems. The main magnet is a very costly item, primarily because of the very high degree of homogeniety required. Also, the radio frequency transmitter requires relatively high power, and must provide this at high uniformity. In addition, regions of inhomogeniety due to susceptibility variations can produce significant distortions. These problems were addressed in U.S. Pat. No. 5,057,776 by the same inventor. Here a pulsed magnet is used to polarize the moments, providing immunity to inhomogeniety. However, except for systems which used radio frequency excitation, this approach did not describe methods for slice selectivity. Of course 3D readouts of the entire volume can be used, but these often require some type of slab selection to avoid aliasing artifacts.